Coating of paper by means of high-pressure coat jets is described in patent applications PCT/FI96/00525 and PCT/FI96/00526. Conventionally, the nozzle used in spray coating is made from a wear-resistant material such as a ceram or ceram-clad steel, for instance. The function of the nozzle is to atomize the ejected coat jet into an aerosol and then to direct this aerosol to the surface of the web being coated. Hence, the nozzle must be capable of spraying the coat in a homogeneous and all-covering manner in order to achieve a high-quality and homogeneous layer of the applied coat.
The coat formulation contains a lot of clay or other solids and the pressure levels employed at the nozzle are high. For instance, the solids may be in the range of about 40-60% and the operating pressure in the order of 100-200 bar (10-20 MPa). Hence, the wear of the nozzle orifices is rapid and their replacement must be performed at short intervals notwithstanding the fact that they are made from the most wear-resistant materials. In the prior art, it has been necessary to replace the entire nozzle which is a clumsy and time-consuming operation. Moreover, the replacement has typically been necessary for a large number of nozzles simultaneously.
For reasons of efficient production, the nozzle parts should be replaceable without stopping the ongoing coating and affecting the coat quality in a noticeable manner. This need requires that the nozzles must be replaceable in small batches, preferably individually. Hence, the replacement of nozzles could be staggered between the nozzles of the coating station, thus assuring smooth and uninterrupted coating despite the nozzle replacements. However, this requirement cannot be fulfilled by conventional techniques or equipment.